1. Field of the Invention
The present invention relates to a telecommunication system using orbiting satellites, and more particularly to a telecommunication system of this kind whose communication channels are increased by using carrier frequency shifts by the Doppler effect.
2. Description of the Prior Art
The development of satellite communication systems up to date has heavily relied on geostationary satellites. Geostationary satellites, which are launched to an altitude of 36,000 km over the equator, require large launching vehicles and accordingly are expensive to launch. Many such satellites have already been launched by a number of countries, leaving a relatively few positions available for additional geostationary orbits. Moreover, since they are extremely far from ground stations and accordingly entail great path loss, they require large-scale transmitters and receivers. For these reasons, conventional satellite communication systems are unable to meet the significantly growing demand for personal communication.
Against this background, telecommunication systems using low-altitude orbiting satellites have come to attract interest. A first advantage of telecommunication using low-altitude satellites is a lower cost per launching attempt because of the relatively small size of the launching vehicle. Therefore, the economic risk of launching failures has been reduced, making it feasible to launch many small orbiting satellites. The limitation on the service areas of low-altitude orbiting communication satellites by their low altitude can be offset by launching a greater number of satellites. A second advantage consists in the ability to provide low-cost store and forwarding service using a few satellites where no real time communication is required (e.g. where communication is to take place between two points having a 180.degree. difference in longitude or a 12-hour time difference). A third advantage derives from the smaller distance of the orbiting satellite than of the geostationary satellite from the ground station, allowing the ground station to be smaller, so that the demand for personal communication can be more readily met.
To provide the aforementioned low-cost store and forwarding service using a small number of orbiting satellites, it is more advantageous to directly link individual ground stations and the satellites by communication lines. If another ground station for the relaying purpose, such as a gateway, is provided, the cost will correspondingly increase. Examples of such a low cost satellite communication system include the ALOHA system having competitive control means (see the Japanese version Of D.W. Davies, D.L.A. Barber, W.L. Price, C.M. Solomonides "Computer Networks and Their Protocols", Aug. 10, 1986, Corona Corp. Japan). Example of a store and forwarding orbiting satellite communication system, using the same competitive control means as in the ALOHA system, include the JAPAN Amateur Satellite-1 (JAS-1), JAS-1b and UoSAT of the University of Surrey, U.K. (see J.M. Radbone, "UoSAT: A Decade of Experience Pioneering Microsatellites", the Symposium International Small Satellite Systems and Service, June, 29-Jul. 3, 1992, Arcachon, France).
In the store and forwarding system using this ALOHA system, since communication requirements in a given geographical area concentrate, along with the orbiting of the satellite, in the visible period of the satellite during which the area comes into the coverage of the satellite (about one hour a day), many more communication channels should be available than in the conventional geostationary satellite-based real time communication system. Thus, if the number of communication demands is the same, the number of communication channels available for the store and forwarding system should be 24 times as great as in the case of the geostationary satellite based real time communication system.
In this store and forwarding system, furthermore, as the distance of the satellite from the ground varies more (6 km/sec), there occur Doppler shifts in receive carrier frequency both at the ground station and at the satellite, and accordingly an allowance for these Doppler shifts should be made in the allocation of frequency channels. In a telecommunication system using an orbiting satellite of 2,500 MHz in carrier frequency placed on a circular orbit of 700 km in altitude, for instance, the Doppler shifts will reach .+-.50 kHz as shown in FIG. 1. Therefore, the required frequency bandwidth for transmitting signals having a bit rate of 9.6 kbps by this communication system, which need not be more than about 20 kHz in the absence of the Doppler shift, should be increased to about 120 kHz, i.e. six times, if an allowance for the Doppler shifts is to be made. Therefore, the number of communication channels available for this communication system will correspondingly decrease. As a solution to this problem, a telecommunication system using a reference pilot signal for compensating for the Doppler shifts is proposed in the specification of the U.S. Pat. No. 4,191,923. In this telecommunication system, each slave ground station detects the Doppler shift amount with reference to the frequency and phase of the reference pilot signal from the master ground station, and corrects the carrier frequency so as to reduce that shift amount to zero. The aforementioned allowance for the Doppler shifts is thereby dispensed with and the number of available communication channels can be correspondingly increased, but many master ground stations would be required. Thus, since the orbiting satellite would need a master ground station in each geographical area matching the satellite-visible period, many master ground stations would have to be installed in different parts of the world. Therefore, it would invite an increase in required hardware for compensating for the Doppler shifts at the ground stations along with an increase in the scale of the telecommunication system, and the overall cost of the system would be thereby boosted.